This invention relates to a variable bearing assembly. More specifically, the subject invention relates to an apparatus for providing a variable vertical and lateral bearing for a support leg of an offshore jack-up platform.
In the past, offshore platforms or towers have been extensively utilized around and upon the continental shelf regions of the world. Examples of offshore platform facilities include supports for radar stations, light beacons, scientific and exploration laboratories, chemical plants, power generating plants, mining stations, etc. Principally, however, offshore platforms have been utilized by the oil and gas industry in connection with drilling, production and/or distribution operations.
In conducting such offshore activity, several platform designs have been utilized by the industry. In deep water applications, semi-submersibles or drillships, which are dynamically positioned and/or turret moored over a well site, have been effectively employed. Although semi-submersibles and drillships are highly mobile and widely utilized in deep water applications, the initial cost and subsequent operating expense reduces the desirability of such units for use in shallow water or intermediate depth applications.
In shallow water situations, fixed length towers or platforms have been extensively utilized. Such platforms are normally fabricated on shore and transported in a generally horizontal posture to an offshore site upon a barge or buoyancy chambers within the platform legs. On site, the platform is pivoted into an upright posture and the base is positioned into firm bearing engagement with the seabed. A platform deck is then fabricated upon the erected tower for conducting offshore operations. Such fixed platforms, although economical in relatively shallow water, require considerable time to assemble and once in position are difficult to relocate.
One platform design which combines many of the advantages of floating and fixed equipment is known as a "jack-up" platform. In this connection a jack-up platform typically comprises a barge or self propelled hull operable to function in a conventional flotation capacity during transportation and as a working deck on location. The hull is fitted with one or more legs which are operable to be vertically extended downward from the deck and into supporting engagement with the seabed.
In operation a jack-up platform is either towed or navigated to a desired offshore site with the jack-up legs extending through wells fashioned through the hull. On site the legs are jacked downward into firm bearing engagement with the seabed. Further jacking serves to raise the hull/deck with respect to the surface of the body of water. Once the lowermost portion of the deck is elevated above a statistical storm wave height, jacking is discontinued and drilling and/or production operations are begun from the elevated deck. Upon completion of the desired offshore operations the deck is jacked down to the surface of the body of water and the legs are jacked up. The platform is then towed or navigated to another working station and the process is repeated. Because of its mobility and versatility, jack-up platforms have emerged as one of the most desirable forms of platform design in the industry.
The subject invention is specifically directed to a jack-up platform variable bearing assembly wherein a novel lost motion footing assembly provides enhanced vertical and lateral stability to a platform leg and in addition facilitates transportation of the platform to a working site and retrieval of the platform legs following working at the site.
The seabed is composed of variant soils and deposits but typically the seabed has a relatively soft upper surface with varying layers of firmer soil strata as the upper layer is penetrated. In some instances relatively loose soils extend downwardly several feet from the surface and in other locations rock or very firm soils lie rather shallow beneath the surface of the seabed.
In order to securely support a jack-up platform in the open sea it is desirable in one sense to provide a fairly large footing at the end of each leg so that vertical loads can be spread over an enlarged area to prevent the leg from penetrating too deeply into the seabed and becoming imbedded. At the same time it is desirable to provide at least a degree of vertical penetration of the leg to enhance lateral stability of the bearing arrangement. Still further it is desirable to be able to tow a platform to a working site in a condition wherein the legs do not project downwardly from the platform hull and thus provide an undesirable hydrodynamic drag.
A significant advance in the art of supporting jack-up platforms was achieved by the conception and development of a tank footing such as disclosed in Moore et al. U.S. Pat. No. 3,628,336 assigned to the assignee of the subject invention. The disclosure of this Moore et al. patent is incorporated herein by reference as though set forth at length. Briefly, however, the Moore et al. patent discloses a tank footing which is releasably connected within a well recessed into the platform hull around each leg. According to the Moore et al. disclosure if the bottom surface is firm the tank footings are not used and they remain connected to the hull. If, however, looser soils are encountered the tank footings are connected to the platform legs in a position above the end of the legs to provide a combination effect of penetration by the leg for lateral stability and vertical bearing over an enlarged footing area. Following operations the leg and footing are jacked back to the hull for transport to a new working site.
Notwithstanding the advantages provided by the foregoing Moore et al. design, room for significant improvement remains. In this regard it would be highly desirable to provide a variable bearing assembly for a jack-up platform that could advantageously be used with a variety of seabed soils while providing enhanced lateral stability for the bearing assembly. Further it would be advantageous to provide a bearing assembly for a jack-up platform which would facilitate removal of the platform leg from the seabed following site operations.
The difficulties suggested in the preceding are not intended to be exhaustive, but rather are among many which may tend to limit the effectiveness and satisfaction with prior platform bearing systems. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that prior jack-up platform bearing assemblies will admit to worthwhile improvement.